ACK Flood attacks are a type of protocol Distributed Denial of Service (DDoS) attack that targets the Transmission Control Protocol (TCP). These attacks exploit the acknowledgment (ACK) packets in TCP communication to overload the target’s network, CPU, or memory resources. By sending a flood of ACK packets, attackers can disrupt normal operations and degrade service availability.
How ACK Flood Attacks Work
In TCP communication, ACK packets are used to confirm the receipt of data. An ACK Flood attack involves sending a high volume of these packets to the target system without any associated data transmission. This creates unnecessary overhead as the target system processes each packet, consuming network bandwidth, CPU cycles, and memory. Attackers often use botnets or spoofed IP addresses to amplify the attack and make mitigation more difficult.
What ACK Floods Look Like in Logs
1. High Volume of ACK Packets
A spike in ACK packets with no associated data overwhelms the target system.
Log Example (Firewall Logs):
[Nov 21 18:00:01] SRC=192.168.1.100 DST=10.0.0.1 PROTO=TCP SPT=12345 DPT=80 FLAGS=ACK
[Nov 21 18:00:01] SRC=192.168.1.101 DST=10.0.0.1 PROTO=TCP SPT=12346 DPT=80 FLAGS=ACK
[Nov 21 18:00:01] SRC=192.168.1.102 DST=10.0.0.1 PROTO=TCP SPT=12347 DPT=80 FLAGS=ACK
[Nov 21 18:00:01] SRC=192.168.1.103 DST=10.0.0.1 PROTO=TCP SPT=12348 DPT=80 FLAGS=ACK
Signs:
- A high number of ACK packets without any prior SYN or data transmission.
- Sudden traffic spikes targeting specific ports, such as port 80 (HTTP) or port 443 (HTTPS).
2. Targeting Specific Ports
Attackers often target open ports to maximize disruption.
Log Example (Access Logs):
[Nov 21 18:01:01] ACK packet received on port 443 from 192.168.1.200
[Nov 21 18:01:02] ACK packet received on port 443 from 192.168.1.201
[Nov 21 18:01:03] ACK packet received on port 443 from 192.168.1.202
Signs:
- Repeated ACK packets targeting the same port.
- Ports with high activity unrelated to normal traffic patterns.
3. Spoofed Source IPs
Attackers may use spoofed IP addresses to hide their identity and amplify the attack.
Log Example (Firewall Logs):
[Nov 21 18:02:00] SRC=10.0.0.1 DST=192.168.1.100 PROTO=TCP SPT=12345 DPT=80 FLAGS=ACK
[Nov 21 18:02:01] SRC=172.16.0.1 DST=192.168.1.100 PROTO=TCP SPT=12346 DPT=80 FLAGS=ACK
[Nov 21 18:02:02] SRC=192.0.2.1 DST=192.168.1.100 PROTO=TCP SPT=12347 DPT=80 FLAGS=ACK
Signs:
- Source IPs appear random or unreachable.
- Patterns in source IP ranges suggest spoofing.